1. Field of the Invention
The present invention relates to an X-ray diffraction apparatus that restricts X-ray on the incident side by a divergence slit and restricts X-ray on the receiving side by a scattering slit.
2. Description of the Related Art
There is widely known an X-ray diffraction apparatus for analyzing the crystal structure of a substance. Some X-ray diffraction apparatus are based on the principle of a focusing method. This principle is sometimes called “Bragg-Brentano focusing method”. As an X-ray diffraction apparatus using a focusing optical system, an apparatus shown in FIG. 7 is known. In this apparatus, an X-ray source F, a sample S, and a receiving slit RS are mounted on a focusing circle Cf. X-ray generated from the X-ray source F is incident on the sample S while its divergence is restricted by a divergence slit DS. When the diffraction condition of Bragg is satisfied, X-ray is diffracted by the sample S, and diffracted X-ray is passed through a scattering slit SS, focused on the focusing circle Cf at the point of the receiving slit RS, and received by an X-ray detector DT.
The divergence slit DS mainly restricts X-ray so that X-ray emitted from the X-ray source F is irradiated on a predetermined area of the sample S. The receiving slit RS mainly prevents extra X-ray (other than X-ray focused on the focusing circle Cf) from being received by the X-ray detector DT. The scattering slit SS mainly prevents X-ray generated from a region other than the sample S (e.g., scattered X-ray generated due to air scattering) from being received by the X-ray detector DT.
There has been known the X-ray diffraction apparatus using a focusing optical system, such as those based on a fixed-divergence angle method and a constant-irradiation width method. In the fixed-divergence angle method as shown in FIG. 7, X-ray diffraction measurement is performed in a state where the slit widths of both the divergence slit DS and scattering slit SS of FIG. 7 are retained constant. FIGS. 8A to 8C concretely explain the fixed divergence angle method. FIG. 8A shows a state where the fixed-divergence angle method is used to measure a low angle region, FIG. 8C shows a state where a high angle region is measured, and FIG. 8B shows a boundary state between the states of FIGS. 8A and 8C.
The boundary state refers to a state where the X-ray width on the sample S defined by the slit width of the divergence slit DS and that of the scattering slit SS coincides with a sample width Ws (e.g., Ws=20 mm) of the sample S when the diffraction angle 2θ is set to a predetermined value (e.g., 2θ=20°). This boundary state occurs under the condition (e.g., slit width d=1° or (½)° in terms of divergence angle, and sample width Ws=20 mm) generally applied to the focusing optical system.
As can be understood from FIGS. 8A to 8C, the slit width d of the divergence slit DS and slit width d of the scattering slit SS are retained at a constant value d=d0 while X-ray incident angle θ changes from a low angle region (<θ1) to high angle region (>θ1). Thus, according to the fixed divergence angle method, the slit width d of the divergence slit DS is always retained at a constant value d0, so that the amount of X-ray supplied to the sample S is kept constant even when the X-ray incident angle θ changes. As a result, the measurement that needs to be performed under the condition that the amount of X-ray is kept constant, such as a quantitative analysis, can be performed with high reliability. However, in the measurement of the low angle region shown in FIG. 8A, the area that is wider than the sample width of the sample S is irradiated with X-ray, so that not only diffracted X-ray from the sample S, but also diffracted or scattered X-ray from a sample holder 101 is generated. Accordingly, the background level in the measurement result is increased to lower the P/B ratio (Peak-to-Background ratio).
The constant-irradiation width method is a method that measures X-ray diffraction while changing the slit widths of both the divergence slit DS and scattering slit SS of FIG. 7. FIGS. 9A to 9C concretely explain the constant-irradiation width method. FIG. 9A shows a state where the constant-irradiation width method is used to measure the low angle region, FIG. 9C shows a state where the high angle region is measured, and FIG. 9B shows a boundary state between the states of FIGS. 9A and 9C. The definition of the boundary state is the same as that described in the above fixed-divergence angle method.
As can be understood from FIGS. 9A to 9C, the slit width d of the divergence slit DS is controlled such that X-ray irradiation width W0 coincides with sample width Ws while X-ray incident angle θ changes from a low angle region (<θ1) to high angle region (>θ1). At the same time, the slit width d of the scattering slit SS is controlled such that the corresponding X-ray width W0 coincides with the sample width Ws. Thus, according to the constant-irradiation width method, the X-ray width on the sample S formed by the slits DS and SS always coincides with the sample width Ws. This prevents the area outside the sample S from being irradiated with X-ray even when the X-ray incident angle θ changes, thereby reducing the background to a lower level. Further, it is possible to increase the intensity of the diffracted X-ray in the high angle region by widening the X-ray irradiation angle. Thus, the constant-irradiation width method is mainly used in a qualitative analysis.
However, although the intensity of the diffracted X-ray is increased in the high angle region, resolution is decreased due to widening of the X-ray irradiation field. Further, the divergence slit width changes with a change of the X-ray incident angle θ and, accordingly, the amount of X-ray that has passed through the slit changes with the change of the X-ray incident angle θ with the result that the amount of X-ray irradiated on the sample S changes. Thus, the measurement that needs to be performed under the condition that the amount of X-ray is kept constant, (such as refined analysis, in general), cannot be performed with high reliability.
There is disclosed, in e.g., Japanese Patent Publication No. 53-28222 (FIG. 2, page 2), an X-ray diffraction apparatus having a configuration in which the slit width of the divergence slit on the incident side is made to change with a change of the X-ray incident angle while the scattering slit is not provided on the receiving side (i.e., the slit width of the scattering slit is fully opened). In this X-ray diffraction apparatus, the slit width of the divergence slit on the incident side changes with a change of the X-ray incident angle θ to thereby prevent the area outside the sample S from being irradiated with X-ray, eliminating the problem that the background level is increased. However, absence of the scattering slit on the receiving side may increase the background level.
Further, there is disclosed, in e.g., Japanese Utility Model Publication No. 3-55890 (FIG. 1, pages 1 to 2), a technique that changes the slit width of the divergence slit on the incident side with a change of the X-ray incident angle θ to thereby retain the X-ray irradiation width on the sample surface constant irrespective of the change of the X-ray incident angle. However, the above publication does not mention whether the scattering slit on the receiving side is provided or not and how to set the slit width of the scattering slit, if provided.
Further, there is disclosed, in e.g., Japanese Patent application Laid-Open Publication No. 2000-55837 (FIG. 1, Page 3), a technique that changes the slit width of the divergence slit with a change of the X-ray incident angle θ within a predetermined X-ray incident angle θa to allow the X-ray irradiation width on the sample surface to coincide with the sample width, while keeps the slit width of the divergence slit when the X-ray incident angle exceeds the predetermined angle θa. This technique intended to prevent the area outside the sample width from being irradiated with X-ray on the low angle side relative to θa so as to reduce the background level and to keep the slit width of the divergence slit on the high angle side relative to θa to prevent the X-ray irradiation field on the sample surface from being widened so as to obtain high resolution.
However, the X-ray diffraction apparatus according to the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2000-55837 could not obtain desired high resolution on the high angle side. The present inventor made various experiments in order to find out the reason for this and, consequently, found out that the reason is that slit control is not made on the receiving side although the slit width of the divergence slit is controlled on the incident side. Although FIG. 2 of Japanese Patent Laid-Open Publication No. 2000-55837 shows a state where some kind of a slit member is provided on the receiving side, the slit member depicted there is a receiving slit used in a typical X-ray diffraction apparatus, not a slit member having a function of restricting the X-ray width on the sample surface. The present inventor did not intend to give a special function to the slit member on the receiving side at the time the application was filed.
Today, demand for an X-ray diffraction measurement is diversified. For example, there is not only a demand for a satisfactory P/B ratio in a low angle region with respect to the diffraction angle (2θ) but also for a high resolution in a high angle region with respect to the diffraction angle (2θ). Such a demand is particularly growing in the field of photocatalysis. A catalyst is generally a substance that speeds up a reaction, but is chemically unchanged itself during the reaction. A photocatalyst is one of those catalysts and a substance that shows the catalytic reaction when exposed to light. Recently, it was found that a slight difference in the crystal structure of TiO2 (titania), which is used as a photocatalyst, leads to a large difference in its photocatalysis function. Thus, it becomes necessary to separate the peaks in a high angle region (e.g., region at which 2θ is set to about 70°) also for management of a production process. Accordingly, there is required an X-ray diffraction apparatus capable of separating TiO2 mixture into respective crystal phases: Anatase (see FIG. 10), Rutile (see FIG. 11), and Brookite (see FIG. 12) and analyzing the component fraction thereof.
FIGS. 10 to 12 are diffraction profiles of respective crystal systems. The respective graphs show different peak values from each other at the region in the vicinity of 2θ=70°. By detecting these peak values, it is possible to determine respective crystal phases. Thus, in order to make this determination, there is a demand for an X-ray diffraction measurement capable of obtaining a high resolution at a high angle region including the region at which 2θ is set to about 70°.
However, the X-ray diffraction apparatuses disclosed in Japanese Patent Publication No. 53-28222, Japanese Utility Model Publication No. 3-55890, and Japanese Patent Laid-Open Publication No. 2000-55837 could not obtain a satisfactory resolution in the high angle region with respect to diffraction angle 2θ. According to the present inventor's consideration, it is assumed that the reason for this is that, in the X-ray diffraction apparatuses disclosed in Japanese Patent Publication No. 53-28222 and Japanese Utility Model Publication No. 3-55890, the amount of X-ray that is irradiated on a sample in the high angle region for controlling the slit width of the divergence slit on the incident side changes, and unnecessary scattered X-ray cannot be eliminated due to absence of the scattering slit on the receiving side.
Further, in the apparatus disclosed in Japanese Patent Laid-Open Publication No. 2000-55837, the slit width of the divergence slit is controlled in the low angle region while the slit width of the divergence slit is kept constant in the high angle region. Therefore, the apparatus was expected to retain a high resolution in the high angle region because the amount of X-ray is kept constant in the high angle region. Actually, however, it was impossible to perform measurement with a high resolution in the high angle region due to absence of the scattering slit for restricting the X-ray width on the sample on the receiving side.
Further, the apparatus disclosed in Japanese Patent Laid-Open Publication No. 2000-55837 adopts a technique that widens the slit width of the divergence slit on the low angle side relative to a predetermined X-ray incident angle θa while keeps the slit width of the divergence slit on the high angle side relative to the predetermined angle θa, which complicates the slit width control of the divergence slit, resulting in an increase in cost.